Video display systems based on spatial light modulators (SLMs) are increasingly being used as an alternative to display systems using cathode ray tubes (CRTs). SLM systems provide high resolution displays without the bulk and power consumption of CRT systems.
Digital micro-mirror devices (DMDs) are a type of SLM, and may be used for either direct-view or projection display applications. A DMD has an array of micromechanical pixel elements, each having a tiny mirror that is individually addressable by an electronic signal. Depending on the state of its addressing signal, each mirror element tilts so that it either does or does not reflect light to the image plane. Other SLMs operate on similar principles, with an array of pixel elements that may emit or reflect light simultaneously with other pixel elements, such that a complete image is generated by addressing pixel elements rather than by scanning a screen. Another example of an SLM is a liquid crystal display (LCD) having individually driven pixel elements. Typically, displaying each frame of pixel data is accomplished by loading memory cells so that pixel elements can be simultaneously addressed.
To achieve intermediate levels of illumination, between white (on) and black (off), pulse-width modulation (PWM) techniques are used. The basic PWM scheme involves first determining the rate at which images are to be presented to the viewer. This establishes a frame rate and a corresponding frame period. For example, in a standard television system, images are transmitted at 30 frames per second, and each frame lasts for approximately 33.3 milliseconds. Then, the intensity resolution for each pixel element is established. In a simple example, and assuming n bits of resolution, the frame time is divided into 2.sup.n -1 equal time slices. For a 33.3 millisecond frame period and n-bit intensity values, the time slice is 33.3/2.sup.n -1 milliseconds.
Having established these times, for each pixel of each frame, pixel intensities are quantized, such that black is 0 time slices, the intensity level represented by the LSB is 1 time slice, and maximum brightness is 2.sup.n -1 time slices. Each pixel's quantized intensity determines its on-time during a frame period. Thus, during a frame period, each pixel with a quantized value of more than 0 is on for the number of time slices that correspond to its intensity. The viewer's eye integrates the pixel brightness so that the image appears the same as if it were generated with analog levels of light.
For addressing SLMs, PWM calls for the data to be formatted into "bit-planes", each bit-plane corresponding to a bit weight of the intensity value. Thus, if intensity is represented by an n-bit value, each frame of data has n bit-planes. Each bit-plane has a 0 or 1 value for each pixel element. In the simple PWM example described in the preceding paragraphs, during a frame, each bit-plane is separately loaded and the pixel elements addressed according to their associated bit-plane values. For example, the bit-plane representing the LSBs of each pixel is displayed for 1 time slice, whereas the bit-plane representing the MSBs is displayed for 2n/2 time slices. Because a time slice is only 33.3/255 milliseconds, the SLM must be capable of loading the LSB bit-plane within that time. The time for loading the LSB bit-plane is the "peak data rate".
A high peak data rate puts high throughput demands on the design of SLMs. To minimize the peak data rate, modifications to the above-described loading scheme have been devised. These loading schemes are acceptable only to the extent that they minimize visual artifacts in the displayed image.
One such modification uses a specially configured SLM, whose pixel elements are grouped into reset groups that are separately loaded and addressed. This reduces the amount of data to be loaded during any one time, and permits the LSB data for each reset group to be displayed at a different time during the frame period. This configuration is described in U.S. patent application Ser. No. 08/300,356, assigned to Texas Instruments Incorporated.